PHT powder coating materials

ABSTRACT

A coating for an article may be made by applying a powder of a polyhemiaminal (PHA) polymer material to the article in a particulate form and then converting the PHA polymer material to a polyhexahydrotriazine (PHT) polymer material and fusing the particles into a monolithic coating by applying heat to the particles. The method generally includes forming a dilute reaction mixture comprising a formaldehyde reactant, a solvent, a primary aromatic diamine, and heating the reaction mixture to a temperature of between about 20° C. and about 120° C. for a short time to form a polymer. A particulate solid is precipitated by adding an excess volume of a non-solvent to the mixture. The powder may be applied to an article and then heated to fuse the powder into a coating and convert the PHA to PHT.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/516,139, filed Oct. 16, 2014. The aforementioned relatedpatent application is herein incorporated by reference in its entirety.

BACKGROUND

Powder coatings are commonly used to form a protecting coating on asubstrate. A powder is applied to a substrate, often using volatileorganic solvents, the solvent is removed, and the powder is cured tofuse the powder particles into a monolithic coating that covers thesubstrate. Powder coatings can extend the lifetime of hardware throughcorrosion protection, abrasion resistance, and protection from chemicalattack.

Currently, polymeric coatings function primarily as passive structures.Their function and value proceed from their mechanical properties. Whenworn or damaged after a time of use, today's polymeric coatings must bereapplied. Moreover, application of powder coatings using organicsolvents is becoming increasingly disfavored as scrutiny increases overuse of such solvents.

A strong coating material is needed that can be applied as a powder,cured into a coating, and subsequently repaired in place without addingadditional material.

SUMMARY

Embodiments described herein relate to powder coatings comprising apolyhexahydrotriazine, methods of making a powder, and methods ofcoating an article. A method of making a powder includes forming a firstmixture of i) a monomer having two or more primary aromatic aminegroups; ii) an optional monovalent monomer having a primary amine group;iii) a formaldehyde reactant; and iv) a solvent, wherein a concentrationof the monomer is about 10 mM or less; heating the first mixture at atemperature of about 20° C. to about 120° C. for a duration less thanabout 10 minutes to form a second mixture comprising a polyhemiaminalpolymer; precipitating a particulate solid from the second mixture byadding a non-solvent to the second mixture; and drying the particulatesolid to form a powder. A method of coating an article includes applyinga powder to a surface of the article, the powder comprising apolyhemiaminal polymer; heating the powder at a temperature betweenabout 190° C. and about 250° C.; flowing the polyhemiaminal polymer toform a coating on the surface; and converting the polyhemiaminal polymerto a polyhexahydrotriazine polymer.

DETAILED DESCRIPTION

A coating for an article may be made by applying a powder of apolyhemiaminal polymer material to the article in a particulate form andthen converting the polyhemiaminal (PHA) polymer material to apolyhexahydrotriazine (PHT) polymer material and fusing the particlesinto a monolithic coating by applying heat to the particles.

A PHA is a polymer that includes a plurality of trivalent hemiaminal(HA) groups having the structure of formula (1)

covalently linked to a plurality of the bridging groups of formula (2)

wherein y′ is 2 or 3, and K′ is a divalent or trivalent radical. In thisdisclosure, starred bonds represent attachment points to other portionsof the chemical structure. The structure of formula (1) is a hemiaminal(HA) group, and each starred bond of a given HA group is covalentlylinked to a respective one of the bridging groups. Additionally, eachstarred bond of a given bridging group is covalently linked to arespective one of the HA groups.

As an example, a polyhemiaminal can be represented herein by formula(3):

In this instance, each K′ is a trivalent radical (y′=3) comprising atleast one 6-carbon aromatic ring. It should be understood that eachnitrogen having two starred wavy bonds in formula (3) is a portion of adifferent hemiaminal group.

The structure of formula (3) can also be represented using the notationof formula (4):

wherein x′ is moles and each bridging group K′ is a trivalent radical(y′=3 in formula (2)) comprising at least one 6-carbon aromatic ring. Itshould be understood that each starred nitrogen bond of a givenhemiaminal group of formula (4) is covalently linked to a respective oneof the bridging groups K′. Additionally, each starred bond of a givenbridging group K′ of formula (4) is covalently linked to a respectiveone of the hemiaminal groups.

Trivalent bridging groups K′ that may be part of a PHA network includethe following structures:

The trivalent bridging groups can be used singularly or in combination.

Polyhemiaminals composed of divalent bridging groups K′ can berepresented herein by formula (5):

wherein K′ is a divalent radical (y′=2 in formula (2)) comprising atleast one 6-carbon aromatic ring. Each nitrogen having two starred wavybonds in formula (5) is a portion of a different hemiaminal group.

Divalent bridging groups usable for the PHA powder material may have atleast one 6-carbon aromatic ring. A category of such divalent bridginggroups may be represented by the structure of formula (6)

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. In an embodiment, R′ and R″ are independently selected fromthe group consisting of methyl, ethyl, propyl, isopropyl, phenyl, andcombinations thereof. Other L′ groups include methylene (*—CH₂—*),isopropylidenyl (*—C(Me)₂-*), and fluorenylidenyl:

Other divalent bridging groups that may be used include:

and combinations thereof.

Non-limiting exemplary monomers comprising two primary aromatic aminegroups include 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA),4,4′-(9-fluorenylidene)dianiline (FDA), p-phenylenediamine (PD),1,5-diaminonaphthalene (15DAN), 1,4-diaminonaphthalene (14DAN), andbenzidene, which have the following structures:

Polyhemiaminals composed of divalent bridging groups of formula (6) canbe represented herein by formula (7):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each nitrogen having two starred wavy bonds in formula (7) isa portion of a different hemiaminal group.

The polyhemiaminal of formula (7) can also be represented by thenotation of formula (8):

wherein x′ is moles, and L′ is a divalent linking group selected fromthe group consisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each starred nitrogen bond of a given hemiaminal group offormula (8) is covalently linked to a respective one of the bridginggroups. Additionally, each starred bond of a given bridging group offormula (8) is covalently linked to a respective one of the hemiaminalgroups.

Monovalent substituents M′ may also be used in the PHA powder material.Such monovalent substituents may have the structure of formula (9),formula (10), formula (11), or formula (12):

wherein W′ is a monovalent radical selected from the group consisting of*—N(R¹)(R²), *—OR³, —SR⁴, wherein R¹, R², R³, and R⁴ are independentmonovalent radicals comprising at least 1 carbon. As noted above, thestarred bond is linked to a nitrogen of a HA group. Non-limitingexemplary monovalent groups having the structures of formulas 4-7include:

wherein the starred bond is defined as above. Monovalent groups can beused singularly or in combination.

Non-limiting exemplary monomers that give rise to monovalent groups in aporous network include N,N-dimethyl-p-phenylenediamine (DPD),p-methoxyaniline (MOA), p-(methylthio)aniline (MTA),N,N-dimethyl-1,5-diaminonaphthalene (15DMN),N,N-dimethyl-1,4-diaminonaphthalene (14DMN), and N,N-dimethylbenzidene(DMB), which have the following structures:

A PHA material having a mixture of mono-, di-, and trivalentsubstituents may have the structure of formula (13):

wherein K′ and M′ are defined as above.

The PHA network may be non-covalently bonded with water through hydrogenbonding, or with another solvent. A PHA complex with water has thegeneral structure of formula (14)

where the stars represent bonds to other atoms in the network, asdescribed above.

A PHA powder material may be made by forming a PHA network in solutionand then precipitating small particles of the PHA network by adding alarge excess of a non-solvent to the solution. A PHA network isgenerally made by forming a reaction mixture of one or more primaryamines with a formaldehyde reactant, which may be formaldehyde, one ormore oligomers of formaldehyde, or a mixture thereof. To form a networkthe one or more primary amines includes at least a primary diamine or aprimary triamine that includes an aromatic group. Thus, a method ofpreparing a PHA network includes forming a first mixture of i) a monomerhaving two or more primary aromatic amine groups; ii) an optionalmonovalent monomer having a primary amine group; iii) a formaldehydereactant; and iv) a solvent. The first mixture is heated at atemperature of about 20° C. to about 120° C. for about 1 minute to about24 hours, thereby forming a second mixture comprising the PHA network.The reaction may be performed using stoichiometric quantities ofaldehyde and amine, or with molar excess of aldehyde. For example, thereaction may be performed at a ratio of aldehyde moles to amine moles ofabout 1:1 to about 1.25:1.

A PHA powder material may be formed using the above process byperforming the reaction under dilute conditions for a short time, suchas less than about 10 minutes, for example about 2 minutes, and thenprecipitating using a large excess, for example a volume of ten timesthe volume of the reaction mixture, of a non-solvent such as methanol.Other precipitating agents that may be used include isopropanol, orother low-boiling alcohols, acetone, or other low boiling ketones,dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and dimethyl formamide(DMF), among others. Dilute conditions are such that no more than about20% solids by weight, for example from about 0.5% to about 20% byweight, are obtained. The powder may be isolated by any convenientseparation method or combination thereof, including filtering,centrifuging, and drying at temperatures less than about 120° C., undervacuum if desired. The powder may have average particle size betweenabout 10 nm and about 100 μm, for example about 50 μm. Average particlesize may be controlled by adjusting the time between combining thereactants and quenching the reaction and precipitating with non-solvent.A shorter time results in smaller particles. Particle size can also becontrolled by adjusting concentration of reactants, with a more dilutereaction resulting in smaller particle size.

The powder may be applied to an article to be coated using anyconvenient means. The powder may be sprayed onto the article using a gaspropellant, flowed over the article by positioning the article on ascreen and dispensing the powder over the article, or by any otherconvenient method. Adhesion of the powder to the article may be enhancedby electrostatic means or by using a compatible adhesive coating appliedto the article prior to exposure to the powder.

The powder may also be applied to the article as an aqueous dispersion,for example as a colloid. A surfactant may be added to the powder toprevent agglomeration in the dispersion, either at the time the powderis precipitated from the reaction mixture or after the powder isprecipitated and/or removed from the solvent. The surfactant may be anionic or non-ionic surfactant. Polyetheramine surfactants, such as theJEFFAMINE® surfactants available from the Performance Products divisionof Huntsman Corporation, located in The Woodlands, Tex., may be used.About 1-2% by weight of a surfactant may be added to the reactionmixture prior to precipitating the powder, and the powder and surfactantmay be co-precipitated. The surfactant may be added to the two-phasepowder-in-solvent mixture after precipitating the powder. The surfactantmay be added to the powder after removal from the solvent mixture.Finally, the surfactant may be added to the aqueous dispersion before orafter adding the powder. After applying the aqueous dispersion to thearticle, the liquid is dried to adhere the powder to the article, andthe powder is then cured as described above.

The powder thus applied to the article may then be cured to form amonolithic coating. The powder may be heated at a temperature of about190° C. to about 250° C. for 30 minutes to 6 hours to cure the powderinto a monolithic coating. In one embodiment, the temperature is rampedslowly up to about 220° C. to cure the coating. During the curingprocess, the PHA network of the powder is converted to a PHT network,and the particles are fused into a continuous coating. Heating thepowder also transforms the powder into a flowable material and flowingthe flowable material along the surface of the article to form acontinuous coating.

A PHT is a crosslinked polymer comprising i) a plurality of trivalenthexahydrotriazine (HT) groups of formula (15):

covalently linked to ii) a plurality of the bridging groups K′ offormula (2), with the starred bonds defined as above, and optionallycovalently linked to iii) one or more of the monovalent groups M′ above.A PHT having a mixture of mono-, di-, and trivalent substituents mayhave the structure of formula (16):

For PHTs comprising bridging groups of formula (6), thepolyhexahydrotriazine is represented herein by formula (17):

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon. Each nitrogen having two starred wavy bonds in formula (11) isa portion of a different hexahydrotriazine group. The divalent groupsdescribed above may be used to make the structure of formula (17). Themonovalent groups described above may also be included.

A PHT may also represented herein by the notation of formula (18):

wherein x′ is moles, L′ is a divalent linking group selected from thegroup consisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ comprises at least 1 carbon and R″comprises at least one carbon.

A coating made from a PHT network is lightweight, rigid, and strong,with Young's modulus of 6 to 14 GPa. The number average molecular weight(Mn) of the PHA and/or PHT polymers can be in a range of 1000 to100,000, preferably in a range of 1000 to 50,000, and most preferably ina range of 1000 to 20,000. The modulus of the polymer can be adjusted byusing more or less monovalent species to control the degree ofcrosslinking.

Mechanical properties of the coating may be adjusted by adding differentbridging groups K′. Plasticity and/or resiliency may be added to thecoating by including thermoplastic bridging groups K′ in the PHA powder.The thermoplastic bridging groups K′ are generally divalentthermoplastic polymer groups of the form *-Q-* each having a molecularweight of at least 1000 g/mole, wherein each starred bond of eachthermoplastic polymer group is covalently linked to a starred bond of ahemiaminal group in the PHA powder, and a hexahydrotriazine group in thecured coating. Such bridging groups may be included in the PHA powder byincluding polymer diamine reactants in the reaction mixture. A usefulpolymer diamine reactant is generally a diamine terminated polymer, suchas a diamine terminated vinyl polymer, a diamine terminated polyether, adiamine terminated polyester, a diamine terminated star polymer, adiamine terminated polyaryl ether sulfone, a diamine terminatedpolybenzoxazole polymer, a diamine terminated polybenimidazole polymer,a diamine terminated epoxy resin, a diamine terminated polysiloxanepolymer, a diamine terminated polybutadiene polymer, and a diamineterminated butadiene copolymer. Diamine terminated polyethers arecommercially available from suppliers such as Huntsman Corp. Diamineterminated vinyl polymers include long-chain alkyl diamines which may bereferred to as polyalkylene diamines, for example polyethylene diamine,polypropylene diamine, and other such polymer diamines. Diamineterminated vinyl polymers also include long-chain polymer diamines withcyclic and/or aromatic components, such as diamine terminatedpolystyrene. The diamine terminated polymers and oligomers referred toabove are commercially available, or may be readily synthesized throughwell-known reaction pathways.

Divalent polymer bridging groups generally have the formula *-Q-*, whereQ is a vinyl polymer chain, a polyether chain, a polyester chain, apolyimide chain, a polyamide chain, a polyurea chain, a polyurethanechain, a polyaryl ether sulfone chain, a polybenzoxazole chain, apolybenimidazole chain, an epoxy resin, a polysiloxane chain, apolybutadiene chain, and butadiene copolymer, or a combination thereof.Typically, a divalent polymer bridging group of these materials willhave a molecular weight that is at least 1000 g/mole. The polymerbridging groups used in embodiments described herein typically come frommixtures of polymer diamine molecules having the general formulaH₂N-Q-NH₂ having a distribution of molecular weights. Molecular weightof a polymer mixture is usually expressed in terms of a moment of themolecular weight distribution of the polymer mixture, defined as

${M_{z} = \frac{\sum{m_{i}^{z}n_{i}}}{\sum{m_{i}^{z - 1}n_{i}}}},$where m_(i) is the molecular weight of the ith type of polymer moleculein the mixture, and n_(i) is the number of molecules of the ith type inthe mixture. M₁ is also commonly referred to as M_(n), the “numberaverage molecular weight”. M₂ is also commonly referred to as M_(w), the“weight average molecular weight”. The polymer mixtures used to obtaindivalent polymer bridging groups in the materials described herein mayhave M₁ of at least about 1000 g/mol.

Molecular weight distribution of a polymer mixture may be indicated by apolydispersity ratio P_(z), which may be defined as

${P_{z} = \frac{M_{z + 1}}{M_{z}}},$where M_(z) is defined above. The polymer bridging groups used inembodiments described herein typically come from polymer moleculemixtures having a polydispersity ratio P₁ of about 1-3, for exampleabout 2.

In one embodiment, the polyaryl ether sulfone polymer of formula (19)for use in forming a PHT or PHA network may be prepared by reacting abis-haloaryl sulfone, a diol such as bisphenol A, and an aminophenolsuch as 1,4-aminophenol in the presence of a base.

The reaction may be performed in a dipolar aprotic solvent such asN-methyl-2-pyrrolidone (NMP), dimethylsulfoxide (DMSO),N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), propylenecarbonate (PC), and/or propylene glycol methyl ether acetate (PGMEA).The sulfone and diol form a polymer terminated by halogen atoms, and the1,4-aminophenol replaces the halogen atoms to leave an amine-terminatedsulfone polymer. The reaction of the sulfone and diol is performed inthe presence of a base, such as potassium carbonate. Molecular weight ofthe sulfone polymer molecules can be controlled by providing a slightexcess of one reactant according to the Carothers equation. Addition ofthe aminophenol stops the polymerization reaction by removing thereactive halide ends.

Other amine-terminated polymers that may be used as precursors for a PHAor PHT reaction include bis-amino polyethers, which are commerciallyavailable or may be prepared by polymerizing an alkylene oxide to apolyalkylene glycol, and then aminating the polyalkylene glycol. A widevariety of reaction pathways are known for producing diamine terminatedpolymers and oligomers.

Coatings made from PHT and PHA polymers also have the property thatscratches and other damage to the coating can be repaired withoutdepositing more material. PHT and PHA polymers can be decomposed tomonomers under acidic conditions with pH<1. A coating that includes PHTand PHA polymers can be treated with a mild acid solution that has pH<1on a surface of the coating to loosen the surface of the coating bydepolymerizing the surface of the coating. If desired, the acid can thenbe neutralized, solvent added to the loosened material on the surface ofthe coating, and the material heated to re-polymerize. The monomers maybe allowed to flow along the surface, smoothing any unwanted features inthe surface and filling any scratches, holes, or other damage beforere\-polymerizing. Alternately, if desired, depolymerized material may beremoved using acid, and replaced with new monomers, and thenre-polymerized. In this way, a coating that includes PHA or PHT polymerscan be repaired.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. When a range is used to express apossible value using two numerical limits X and Y (e.g., a concentrationof X ppm to Y ppm), unless otherwise stated the value can be X, Y, orany number between X and Y.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and their practical application, and toenable others of ordinary skill in the art to understand the invention.

What is claimed is:
 1. A method of coating an article, comprising:applying a powder to a surface of the article, the powder comprising apolyhemiaminal polymer; heating the powder at a temperature betweenabout 190° C. and about 250° C.; flowing the polyhemiaminal polymer toform a coating on the surface; and converting the polyhemiaminal polymerto a polyhexahydrotriazine polymer.
 2. The method of claim 1, whereinthe polyhemiaminal polymer comprises: a plurality of trivalenthemiaminal groups having the structure

 and a plurality of divalent linking groups wherein at least one starredbond of each hemiaminal group is covalently linked to a respective oneof the divalent linking groups, and each divalent linking group iscovalently linked to two hemiaminal groups.
 3. The method of claim 2,wherein each divalent linking group has the structure

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon, and each starred bond of each divalent linking group iscovalently bonded to a respective starred bond of a hemiaminal group. 4.The method of claim 3, wherein the polyhemiaminal polymer furthercomprises a plurality of monovalent groups, wherein each monovalentgroup is covalently linked to a hemiaminal group.
 5. The method of claim1, wherein the polyhexahydrotriazine polymer comprises a plurality oftrivalent hexahydrotriazine groups having the general structure

and a plurality of divalent linking groups, wherein at least one starredbond of each hexahydrotriazine unit is covalently linked to a respectiveone of the divalent linking groups, and each divalent linking group iscovalently linked to two hexahydrotriazine groups.
 6. The method ofclaim 5, wherein each divalent linking group has the structure

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon, and each starred bond of each divalent linking group iscovalently bonded to a respective starred bond of a hexahydrotriazineunit.
 7. The method of claim 6, wherein the polyhexahydrotriazinepolymer further comprises a plurality of monovalent groups, wherein eachmonovalent group is covalently linked to a hexahydrotriazine group.
 8. Amethod, comprising: forming a first mixture of i) a monomer having twoor more primary aromatic amine groups; ii) an optional monovalentmonomer having a primary amine group; iii) a formaldehyde reactant; andiv) a solvent, wherein a concentration of the monomer is about 10 mM orless; heating the first mixture at a temperature of about 20° C. toabout 120° C. for a duration less than about 10 minutes to form a secondmixture comprising a polyhemiaminal polymer; precipitating a particulatesolid from the second mixture by adding a non-solvent to the secondmixture; and drying the particulate solid to form a powder.
 9. Themethod of claim 8, wherein the powder has an average particle sizebetween about 10 μm and about 100 μm.
 10. The method of claim 8, furthercomprising applying the powder to a surface of an article, and fusingthe powder into a coating by heating the powder.
 11. The method of claim10, wherein heating the powder comprises heating the powder at atemperature of about 190° C. to about 250° C., forming a flowablematerial from the powder, and flowing the flowable material along thesurface of the article to form a continuous coating.
 12. The method ofclaim 11, wherein the powder comprises a polyhemiaminal polymercomprising a plurality of trivalent hemiaminal groups having thestructure

and a plurality of divalent linking groups wherein at least one starredbond of each hemiaminal group is covalently linked to a respective oneof the divalent linking groups, and each divalent linking group iscovalently linked to two hemiaminal groups.
 13. The method of claim 12,wherein the continuous coating comprises a polyhexahydrotriazine polymercomprising a plurality of trivalent hexahydrotriazine groups having thegeneral structure

and a plurality of divalent linking groups, wherein at least one starredbond of each hexahydrotriazine unit is covalently linked to a respectiveone of the divalent linking groups, and each divalent linking group iscovalently linked to two hexahydrotriazine groups.
 14. The method ofclaim 13, wherein each of the polyhemiaminal polymer further comprises amonovalent group covalently linked to a hemiaminal group, and thepolyhexahydrotriazine polymer further comprises a monovalent groupcovalently linked to a hexahydrotriazine group.
 15. The method of claim14, wherein each divalent linking group has the structure

wherein L′ is a divalent linking group selected from the groupconsisting of *—O—*, *—S—*, *—N(R′)—*, *—N(H)—*, *—R″—*, andcombinations thereof, wherein R′ and R″ independently comprise at least1 carbon, and each starred bond of each divalent linking group iscovalently bonded to a respective starred bond of a hexahydrotriazineunit.